feature: Reuse existing organization, organizational units and accounts

[jeromevdl]

Issue #, if available: #13

Description of changes:

• the bootstrap kit now checks if there is already an existing organization before trying to create one. If it exists, it will use it, otherwise will create one.
• the bootstrap kit can now reuse parts or the complete account hierarchy (organizational units). If using the same OU name under the same parent, OU will be reused, otherwise it will be created.
• the bootstrap kit can now reuse existing accounts (using a flag 'reuseAccountId' in the nestedOU structure). Existing accounts can be move to another OU.
• Add resiliency by removing some 'throw' in CDK (for example in "Delete" events), to avoid failures in CloudFormation.

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[jeromevdl]

A lot in common with https://github.com/pepperize/cdk-organizations, maybe use this lib at some point. WDYT?